Great progress has been made in developing of catalysts for olefin metathesis reactions (including stable and active catalysts, such as formula A and B). It allows application of metathesis to synthesis a large number of compounds.

Removing heavy-metal containing impurities from reaction products may be important for the introduction of olefin metathesis in pharmaceutical industry. The development of an efficient, economical and practical method to remove metal by-products may help to further application of metathesis methodology.
Several scavengers have been proposed to remove residual Ru from reaction mixtures or from crude products. There is no universally attractive method so far. The protocols mentioned above were used when classical, non-tagged catalysts were applied.
In order to develop new processes for the removal of ruthenium from reaction products, a few immobilized catalysts which contain an onium group have been synthesized. It was shown that a simple purification step resulted in low levels of residual Ru impurities in the crude products. However Ru levels may still be too high for pharmaceutical application.
There is only limited number of complexes bearing quaternary ammonium groups known in the literature. This may be due to complicated synthesis and purification of such a complex. Alternatively, polar, uncharged complexes, with high affinity to adsorbents can be applied to facilitate residual Ru removal. For example complex C with high affinity to silica gel was synthesized. Application of C in metathesis resulted in products with significantly reduced residual ruthenium after a simple purification step.

However, the results obtained with C were still far from the requirements of the pharmaceutical industry (residual Ru in the product ranged from 83 to 420 ppm).
Complexes bearing amido groups such as D and E were synthesized as well.

However, in case of reactions catalysed by D or E the possibility of residual Ru removal by simple filtration of reaction mixture through a small amount of adsorbent was not demonstrated.
Activity and efficiency of complexes described herein are comparable with those observed for catalysts known in the literature. It was unexpectedly found that residual Ru can be simply and efficiently removed from reaction mixture or from crude product using an inexpensive purification method when complexes bearing hydroxamic acid ester group are applied as a catalysts.